1. Field of the Invention
The present invention relates to a directional loudspeaker unit. More specifically, the present invention relates to a loudspeaker unit intended to realize directivity suitable for enlarging a region of satisfactory stereophonic listening position by attaching a first loudspeaker driven in a positive polarity to a front panel of a loudspeaker box, attaching a second loudspeaker driven in a negative polarity to, for example, an upper panel of the loudspeaker box, and driving the first and the second loudspeaker so that the radiation sound pressure of the second loudspeaker is lower than that of the first loudspeaker.
2. Description of the Related Art
The following are directional loudspeakers according to the prior art.
FIG. 13 shows a Tonsaulen directional loudspeaker unit 20 by way of example. The loudspeaker unit 20 has a plurality of loudspeakers, i.e., five loudspeakers 22a to 22e, linearly arranged on a panel of a loudspeaker box 21. The loudspeaker unit 20 provided with the laterally arranged plurality of loudspeakers has a sharp directivity in a horizontal plane.
FIG. 14 shows a bidirectional loudspeaker unit 30. This loudspeaker unit 30 is assembled by attaching two loudspeakers 32a and 32b to the front and the back surface of a baffle plate 31 opposite to each other. The two loudspeakers 32a and 32b are driven respectively in opposite polarities.
A loudspeaker unit according to the present invention, which will be explained later, is different in construction and directivity from the foregoing conventional directional loudspeaker units.
As shown in FIG. 15, a conventional two-channel stereophonic loudspeaker unit 40 has a right loudspeaker 41R attached to a front panel of a loudspeaker box 42R, and a left loudspeaker 41L attached to a front panel of a loudspeaker box 42L. The loudspeaker boxes 42R and 42L are disposed with their front panels facing a listener situated in front of the loudspeaker unit 40. When listening to stereophonic sounds generated by the loudspeaker unit 40, listening positions suitable for satisfactorily listening to stereophonic sounds are limited to those in a very narrow region including a point a on a center line bisecting a line connecting the loudspeakers 41R and 41L as indicated by a shaded region in FIG. 15.
Distances from a listening point b not on the center line to the right and the left loudspeakers are different from each other, sounds generated by the right loudspeaker 41R can be heard louder than those generated by the left loudspeaker 41L. Therefore, at the listening point b, sound image localization is biased toward the right loudspeaker 41R and hence a natural sound stage for two-channel stereophonic sounds cannot be created.
A conventional two-channel stereophonic sound reproducing method intended to enlarge a range for listening positions where satisfactory stereophonic listening is possible utilizes the directivity of loudspeakers.
A two-channel stereophonic loudspeaker unit 50 shown in FIG. 16 is provided with a right loudspeaker 51R and a left loudspeaker 51L attached to closed cabinets 52R and 52L with their reference axes directed inward at an angle of about 45.degree. with respect to a center listening position to use directivity dependent on the diameters of the loudspeakers, and the dimensions and shapes of the cabinets.
This loudspeaker unit is intended to enlarge a region for listening positions where stereophonic sounds can be satisfactorily heard by the corrective agency of the directivities of the loudspeakers to reduce the difference between the respective levels of an R signal sound and an L signal sound attributable to the difference between distances from a listening position b dislocated from the center line to the loudspeakers.
Generally, directivity of a loudspeaker is dependent on the diameter of the loudspeaker, and the shape and dimensions of the cabinet, and a loudspeaker of a smaller diameter becomes directional for sounds of higher frequencies and becomes substantially non-directional for sounds of middle and low frequencies. Directivity for sounds of frequencies not higher than 1 KHz affects greatly appropriate listening region enlarging effect. In FIG. 16, a curve w1 indicates a directivity pattern for sounds of middle and high frequencies, a curve w2 indicates a directivity pattern for sounds of middle and low frequencies, and a indicates, similarly to a in FIG. 15, a point on the center line bisecting a line connecting the right and the left loudspeaker.
Directivity capable of enlarging a region for listening points must decrease sound pressure with the increase of the angle of a direction to the reference axis of the loudspeaker from 0.degree. toward 90.degree. as indicated by a curve w3 in FIG. 17, and it is desirable that the sound pressure with respect to a 90.degree.-direction is lower than that with respect to a 0.degree.-direction by 6 dB or above.
Accordingly, it is an object of the present invention to obtain directivity which makes the sound pressure of sounds of frequencies not higher than, for example, 1 KHz in the 90.degree.-direction lower than that in the 0.degree.-direction (axial direction) by 6 dB or above, i.e., directivity suitable for enlarging a region for satisfactory stereophonic listening positions.